The present invention relates to a downhole-hammer drill-bit assembly. More particularly, the present invention relates to a self-retaining drill-bit assembly having a tubular retainer with a split sidewall that aids in the retrieval of a drill-bit that breaks or shanks-off during drilling.
Cyclic fatigue induced crack nucleation and propagation to fracture is a common failure mode for hammer bits. Typically, the fracture occurs in the shank below the retaining ring and above the bit head and results in a complete separation of the bit head from the shank. Recovery of the severed bit head from the bottom of a drill hole often requires a fishing procedure the outcome of which is uncertain and adversely impacts drilling productivity.
Current retention devices typically have a generally one-piece cylindrical collar or sleeve that is placed around a drill-bit shank and that extends axially above and below the portion of the shank where fractures are known to occur. The retention devices are usually attached to various structural elements of the drill bit and drill chuck or drill casing by locking pins or snap rings that can loosen or shear causing damage to a drill string.
A retention device, such as the present invention, that is self-retaining, thereby reducing the number of parts comprising the device and eliminating the potential for damage caused by loose or sheared parts is a significant advancement in downhole-hammer drill bit retention devices.
One aspect of the present invention relates to a downhole-hammer drill-bit assembly for removable attachment to a substantially tubular casing of a downhole hammer. The tubular casing has an axially extending casing bore with an inner diameter. The drill-bit assembly comprises a substantially tubular chuck, a drill-bit and a substantially tubular retainer. The substantially tubular chuck has an axially extending chuck bore and a generally radially outwardly facing surface with a first portion, a second portion, and a step between the first and second portions. The first portion is insertable in the casing bore and removably attachable to the casing. The step has an axially-extending outwardly-facing first surface having an outer diameter less than the inner diameter of the casing bore and a radially outwardly-extending second surface having an outer diameter greater than the inner diameter of the casing bore. The drill-bit has a bit shank with an outer diameter and a bit head. The bit shank has a radially outwardly-extending bit shoulder having an outer diameter greater than the outer diameter of the bit shank. At least a first portion of the bit shank is in the chuck bore. The substantially tubular retainer has a split retainer sidewall, a first end and a second end. The split retainer sidewall is around the second portion of the chuck and around the bit shoulder. The first end has a radially inwardly-extending retainer shoulder with an axially-extending first lip forming a split retainer collar having an outer diameter less than the inner diameter of the casing bore and an inner diameter greater than the outer diameter of the first surface of the step. The split retainer collar is around the first surface of the step. The second end has a radially inwardly-extending second lip forming a split retainer ring having an inner diameter less than the outer diameter of the bit shoulder. The split retainer ring is around a second portion of the bit shank between the bit shoulder and the bit head.
Another aspect of the present invention is a method for retaining a severed downhole-hammer drill bit, comprising the steps of inserting a drill bit in a bore of a tubular chuck, placing around a portion of the chuck and a portion of the drill-bit a tubular retainer having a split sidewall, a split ring and a split collar such that the split ring is below a shoulder of a shank of the drill bit and the split collar is above the shoulder and securing the chuck to a drill casing such that the split collar is retained between a radially outwardly facing surface of the chuck and a radially inwardly facing surface of the drill casing.